Telegraph repeater



Jan. 26, 1954 A. MAHoNEY ETAL TELEGRAPH REPEATER 6 Sheets-Sheet 1 Filed July 6, 1951 m021 winx @v N m um um n MRSE A /.BIM Jwr..d

W y W B w Mim NG m ud. w

J. A. MAHoNEY x-:rAL 2,667,537

TELEGRAPH REPEATER Jan. 26, 1954 G'Sheets- Sheet 2 Filed July 6. 1951 NCI-Q 6 Sheels-Sheerl 4 TELEGRAPH REPEATER J. A. MAHONEY ET AL sla-ill ai m, ...QM

Jan. 26, 1954 Filed July 6, 1951 Jam 26, 1954 J. A. MAHONEY ET Al. 2,667,537

TELEGRAPH REPEATER Filed July 6, 1951 6 Sheets--SheerI 5 DIS TAN T ROA R808 RQOA R/OOA '.J. A. MAHONEY /NVE/v ToRs m T. REA

By c. B. su TL/FF A T TORNEV Jan. 26, 1954 J. A. MAHONEY ET AL 2,667,537

TELEGRAPH REPEATER Filed July e, 1951 l v e sheets-sheet e B48 RES A T TORNE V Patented Jan. 26, 1954 TELEGRAPH REPEATER Joseph A. Mahoney, New York, and Wilton T. Rea,

Manhasset, N. Y., and Carleton B. Sutliff, Summit, N. J., assignors to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application July 6, 1951, Serial No. 235,458

11 Claims. 1

This invention relates to telegraph systems and more particularly to an improved telegraph repeater.

The repeater may for purpose of description be 4considered to comprise a line side which extends to a distant repeater station and a local side. In the present arrangement the line side may be quickly changed, by the manual operationof rotatable switches, so that the type of transmission between the present line repeater and a distant line repeater may be any of three types, namely, differential duplex, type A polarential and type B polarential.

The line repeater when in the differential duplex condition may be arranged for either normal or upset differential duplex operation.

The line repeater when in the type A polarential condition may be arranged as either the polar sending terminal or as the diiierential sending terminal.

The line repeater when.A in the type B polarential condition may be arranged as either the polar sending terminal or as the differential sending terminal.

' The line between two repeater stations may z be subject to different kinds and degrees of inductive disturbances. The present line repeater can be arranged through the operation of a rotatable switch, the Line switch, in three diierent manners to care for three different induction conditions.

In the rst of these induction conditions the disturbance on the line due to induction is negligible and a single wire line is used to interconnect the two repeater stations and no induction suppression elements are inserted in the line.

In the second of these induction conditions the line between repeater stations is subject to (S-cycle inductive disturbance, the line between stations is arranged as a single wire line and elements are connected to the line to suppress 60-cycle currents.

In the third of these induction conditions the facility between repeaters is a two-wire line and the repeater is arranged so that all longitudinal disturbances are balanced out.

The line repeater of the present invention may be connected on the local side in two diierent manners. In the first manner of connection the line repeater is connected through a balanced loop circuit. In the second manner of connection the line repeater present is connected through an electronic coupling unit, or as it is otherwise known through an electronic control circuit, for connection to a hub-type repeater system. Y

When the local side of the present repeater is a balanced loop circuit, the present repeater may be extended on the local side in two diiTerent manners. First, it may be connected to a sub- 'criber loop circuit. Second, it may be connected back to back with another balanced loop and line repeater such as that to be described herein to form what is termed a through repeater, which is an intermediate repeater in a tandem connection extending to a first distant repeater station, say to the East, and a second distant repeater station, say to the West. In this latter arrangement, at the through or intermediate point, a rst line repeater is connected through its associated balanced loop to form one repeater which is connected through a second balanced loop and its associated line repeater forming the second repeater so that two line repeaters and two balanced loops such as those disclosed herein are provided at the through point. In this arrangement each of the balanced loops may be terminated in a test board. Alternatively when serving as a through or intermediate repeater one line repeater and one balanced loop such as that to be described herein may be employed and instead of a second line repeater and a second balanced loop such as that to be described herein another suitable balanced loop type repeater may be employed.

When the line repeater and balanced loop circuit described herein are interconnected, the local side of the circuit may be arranged in two different manners. It may be arranged for fullduplex operation or for half-duplex operation. In both the full-duplex and half-duplex arrangement the battery connections may be normal for one condition of operation or reversed for another condition oi operation.

To facilitate changing from one to another of the various arrangements possible when the local side of the repeater is a balanced loop, the

3 three different transmission arrangements of the line extending to a distant repeater station are possible, namely, differential duplex, polarential A and polarential B. The line repeater is provided with two manually rotatable switches which cooperatively arrange the circuit for any one oi the three options. These switches are called herein option switches and the two provided are identified on the drawings as the OPNi and the OPN2 switches. The switch which adjustsV the line repeater for the different induction conditions of the line is identified on the drawings as the Line switch. In addition to these there are three other,l manually rotatable multiunit switches shown on the drawings in the line side of the repeater which are employed to make various adjustments, which switches will now be described generally.

The rst of these three switches is the apex switch, designated Apex on the drawings.` It is s employed to vary the resistance connected between the armature of the sending relay S and the apex of the receiving relay R, which is the junction between the path extending through the line winding of thereceiving relay R-to the line and a common shaft. The reason for varying the re- 'i sistance in the apex circuit will be made clear hereinafter.

A second of thethree other rotatable switches in the line side of the repeater is the line pad switch or L Pad switch as it is designated on the drawings. rIfhere are foursections on the L Pad switch. Each section primarily comprises al resistance variable in discrete xed amounts. One adjustable resistance may be connected in series with the transmission line. connected in series with the balancing line for the transmission line. The third is connected in series in the second or neutralizing wire when used and the fourth in a balancing circuit for the second wire. All four resistances are variable in discrete equal amounts in unison and their purpose may be understood from the description hereinafter.

' YIhe third of the rotatable multiunit switches in the line side of the repeater is the capacitance balance,y or C Bal switch as it is designated on the drawings. It is arranged to connect a variable amount ci capacitance in the balancing network and a corresponding amount-in the neutralizing line when the neutralizing line is employed. It is a four-unit switch, two units associated with the balancing line and two with the neutralizing line. It aords zero capacitance or any one of eleven diierent amounts of capacitance in unit v The second may be 4 received signal in comparison with that of the higher frequency components in order to minimize the distorting eiects of the line characteristics on the signal during transmission.

TheseV and other featuresof the invention will become apparent from the following description when read with reference to the associated drawings in which Figs. 1, 2 and 3 taken together and disposed as in Fig. 4 disclose the improved repeaterl ofthe present invention with its local side arranged as a balanced loop.

Figs. 5; 6 andV 7 taken together and disposed as in Fig. 8l disclose: the improved repeater of the present invention with its local side arranged as r an electronic coupling unit for connection in a hub repeater system.

InY the. following description where values of constants are given, it is to be understood that the values cited are by way. of example.

Refer now to Figs. 1, 2 and 3 disposed asin Fig. 4.

Where the same conductor extends between two drawings, it is designated with the saine letter on each drawing.

First the multicontact, multisection switches shown in Figs. 1, 2 and 3 will be described.

The circuit is equipped 'with seven multisection, multicontact switches of a type well known in the art. In Fig. 1, one'of these switches is shown designated Loop. Itecomprises two switch sections designated A and B.- Each switch section comprises a set of twelve contacts numbered l to i2 arranged in a circle about inner arcuate conducting segments. The twelve contacts project radially inward-1y. The twelve contacts are stationary. They are of two different lengths. The arcuate segments are rotatable. In the normal or zero position of the switch the rotatable arcuate conducting segmentsl are in the positions indicated. The arcuate segments of each of sections A and B may be considered to be integrally mountedV on the saine shaft and are rotatable in unison, step by step, tofour discrete positions, numbered 1 to 4, as shown in Table G hereinafter. The rotatable segments are of diierent widths measured radially. The longer of the contacts will make engagement with a portion of a rotat-Y able conducting segment of either width when in radial alignment therewith. The shorter of the contacts will make engagement onlyA with a por tion of a segment of greater width when in radial alignment therewith. 'v

Reference tosection AA ofY the Loop switch shows that when the switch is in the positionf inffv dicated on the drawing, which corresponds tothe zero position, as shown in Table G, contacts, IV and l) ,are interconnected.; contacts 2, 4 and 6 areinterconnected; and contacts 7 and Sare inf terconnected. The othercontacts of switch unit Aare notinterconnected.. Reference to switch section B of the Loop switch ,shows that, with the switch in the same zero position, contacts .2 and 3 .areinterconnectedg contacts 4 andA E .are interconnected; and contacts 9 and H are interconnected. YThe other contacts are not. intercon-v nected.

The switch is rotated.A intoA the next discrete position, that is into position 1, as shown in Table G hereinafter, by moving the shaft clockwise through a YSO-degree angle. In thisV position in the case of section` A, contacts I, 2 and ll are interconnected; contacts 5 andl tare intercom:

nected; and contacts 8 and Q areinterconneeted In the case of section Bcontacts..andiare interconnected; contacts and 6 are interconnected and contacts 9 and Il are interconnected. The contact connections which are established through section A and through section B of the Loop switch when the Loop switch is in each one 5 of its four discrete positions may be understood from the foregoing and are shown in Table G for each of the four positions 0, l, 2 and 3.

The manner of operation of each of the othei` six 'multisection, multicontact switches may beV understood from the foregoing. In Fig. 2 four other such switches are shown each having two individual sections, sections A and B. These are designated Apex, Line, OPN i and OPN2, respectively. In Fig. 3 two more such switches are shown designated L Pad and C Bal each of which has four'sections A, B, C and D, respec-V tively. As indicated in Tables A, B and C, respectively, the OPNI, OPNZ and Line switches TABLE E C Balance Swtich Contacts Connected Switch Position n Sec. A Sec. B Sec. C Sec. D

, Y A em each have three positions 0, 1 and 2, respectively. 20 p Table D shows that the switch designated L Pad S .t h Co'nta ts has eight positions. designated 0 to 7, and as Wnnectedc shown in Table E and F, respectively, the switch Switch Position Condition designated C Bal and the switch designated Apex Sec, A Sem B each has twelve positions designated 0 to 11.

o 9-10 9-10 TABLE A 23s n; ai

4 OPN soo 1 9 1-9 13% it i3 Switch contacts connected 11200 3:11 3-11 1, o 3-12 3-12 Switch position Condition 1y 600 1 3 1 3 Sec. A See. B 1,800 2-3 2-3 2,000 gone gone PB i-2, se, 9-10 i-2,7-s,9-io 2000 one one PRA 1 2, 5 6, 9-10 nas-9,1041 35 PSA 1-4, 5-8, 9-12 1-4, 7-8, 9-10 TABLE G TABLE B Loop switch,

OP N 2 s t h Switch Contacts Connected Wl C n 40 Position Condition s .t h Switch contacts connected Sec. A Sec. B

Wl C position Condition Sec. A Sec. B 1-10, 2-4-6, 7-9 2-3,45, 9-11 ...cani-ii DX 9-10 1-12,3-4,6-7,9"io ,f

PS 1 2, 9 10 None 4 i s i 4,5-6-7 9 i ii,a-4,5 6,8 e PR 4-5, 7-8, 9-12 None y The line side of the repeater is identical in each TABLE C Line Switch contacts connected Switch Line wires Interference position connected condition Sec. A Sec. B

1 Negiigibie 2 4, 9-11 i-1o,a5 i so cyeies asa-5,1142 i-2-io-1i,45c 2 A11 iongimainaL-. 1-2-11, 7-8, 9-10 1-2-3-12, 5 7, 8 9

TABLE D of the arrangements per Fig. 4 and Fig. 8. The line side of the repeater in the arrangement per L Pad Fig. 4 comprises the right-hand portion of Fig. 1 Switch contacts connected and all of Figs. 2 and 3 which corresponds inthe Switch arrangement of Fig. 8 to the right-hand portion position condition Sec Sec of Fig. 5 and all of Figs. 6 and 7. The left-hand sea. Seite C' D- portion of Figfl shows the so-called local side' of the repeater, which in Fig. 1 is arranged as 8% guom 4 3, 9 10 4 6, 3:10 6 1@ 6 1@ a balanced loop. Correspondingly the left-hand u 15 2- ,9-11 2-3, 1i 61o s-io o CX OutBoom 24,1041 2471041 6 10 6 10 ,0 p rtion of Fig. 5 shows the local side, whichni 8% gummi" 1L-6% Ngone 6 10 6 10 Fig. 5 is arranged as an electronic coupling oir- 4 6, -io 2-10 2-10 c CX In 150m" 2 3941 2 3, 9 11 2 10 2 10 uit for connection to a hub repeater system. CX In Boon"A 24,1041 2 4y10 11 2 1() 2 1() The type 0f 106341 Clrcult t0 Whlch. the Slde ox In 45o... None None 2-10 2-10 of the repeater is connected depends upon the method oi operation and interconnectionV employed in an ofiice. Eachfotthe local terminating circuits which caribe u Sed with the line side of the repeater is arranged to control the sending relay YS in theV line sideof the repeater in Fig. l' for transmission of signals and to receive signals fromrelay R armature. Subject to certain limitations transmission performance which are to be disclosed later, any of the local terminations cangbe usedv with'any ofthe types of transmissionV oreany of- .the line. arrangements fplovidedfoln. the repeater.

rIYRANSMISSION OF SIGNALS (3V-EFE?.v 'Ill-Hit The following description Vof transmission oven the lineis written as though the repeater were operating to another repeaterof the same type at the other end of the line', but it is equally applicable when the repeater -operates to any other suitable repeater or teletypewriter subscriber set. The-*total current in each branch of the repeater circuiti at each terminal includes components which aresupplied by the home or sending batteryand other components which are supplied by the batteries at thev remote'repeat'er. Operation of thefrepeater is described herein by considering separately the effects of current compo-- nents duel to thefhome battery and those vdue to the remote battery;-y

Operation of the; armature of relay S underv control of the local terminating circuit associated with the repeater.y applies marking or spacing battery conditions to the line-through the apex-circuit and one winding of relay R. A similar magnetically opposing winding of relay R is connected in a circuit from the .apex branch through the linembalanoing network to ground. When adjusted for duplexv balance, the network impedance. closely matches that of the une, so that signal currents supplied to the line under control orthearmature of localielay are matched by equal currents in the network branch and relay Rissubstantially unaffected.

Signal current components in the local re- .1

peater due to the battery conditions supplied to the line by relay vS in the remote repeater may be called f 'received' signal currents. These-f currents iiow through the line winding of relay R and to ground, part through the apex branch including contacts of relay S and part through the balancing network. The line winding and the networkV Winding of relay R are series-aiding with respect to received signal currents and are poled solas .to recognize as marking or spacing the current directions corresponding to the position of. the

armature of relay S in the remote repeater. Inf

the repeater is in duplex balance as described in the last preceding paragraph, relay R will be controlled by the received signal currents while being unaiectedby ,the transmission of outgoingL sig-i nttlSl` Anequalilzger consisting of a coil inV series le resistance caribe connected across I '1" receiving relay R,`1no.difying,tlie curr rit,waye shape inthe vrelay soY thatsoine forms't ofj -signal distortions are reduced. The

' izr isdescribed in detail hereinafter,

t types v,of transmission over the lineare provided-known as differential duplex, type Ay.. polarential and type B polarential "They durer in'the, .battery conditions supplied to the contacts. Otr'elay'S ,and in the-winding connections of relay Rf Av vhlclrin each case must be arranged forH Draper operation in response tothe battery C911: eiiinssnplibrih ramde repeats? Differ:

- aac@- Eah type O f-Srstem is capable-0 von. theloal, Side 0r With any. aff thefinc .'airied.,Y full-duplex operation., Orsimultan used with anyof thefermina'tirls afwas nl arrangements of the lineside, butv the charac iSticS. 0f, .each System, as. descibed nfdetail.- here? inafter.,i..maka.it .beet suited t0. meeting, Certain Operating requirement Differential* (111121 f perati9r1v mar ,be used when the l duplexV bala can bireadjusted. as required, br chang.

Conditions Ifl ahieh degree. @fg-balance isf mai opera-tion in both directions can-,be provide Polarerltialv transmission is. intended *fare y11S where-the Asen/ice chair-duplexes@ freedom from readiustmentis. desired- The type A-:sys tem should be usedwherefitismore important minimize-the efects Of lrleresistancallane, a., and the type `B System Should :be l1-.Sed Yrifhclrefit,- is more important @minimize thecfietsf-lioaf leakage. A

The repeater:visV conditioned for; Operationeea terminal of a differential duplex or polarential system by operation of switches OPN I and OPNZ to positions indicated by the tables on the drawing. Switch OPN l controls the supply of positive and negative 15G-volt battery to the contacts ofrrelayS. and arranges the windings, OfrelayB so as to recognize either positive or negative battery at the remote-repeater as marking.4 SwitchH OPNZ ycontrols, an alternate supply of VgIOuIld stead of battery tothe marking contacts of relay, Sl and opens' or closes the bias current'circuit of relay R. Short circuits. areapplied to the ad iustable line pads and apex resistance for terminal conditions which rduire a zero setting of these controls. The connections provided by the y switches OPN l and OPNZ are described in detail hereinafter The repeater canbe arranged under. control of kthe Line switch'for linei co'nnectionswhich provide for reducing the adverse eiiects of 'different kinds anolgdeg-rees of line interference. Any of the transmission systems may be used with any of the line arrangements. If the total line interference is relatively small, .a single telegraph line circuit, called hereinafter line I, may be used andthe. Line switch should bein position 0. The effects off(iO-cycle*powerinductance into a one`` wiresystem may bereducedqby atunedv shunt which iscOnneCted acrossl the windings of relay Y R when! the VLine switchisinI position 1. A highw degree of protection against '60icycle or other kinds'of line interferences,` including that dueto earth potentials and'telegraph Crossfire, may be obtained by the use of'asecond or neutralizing line. Operation of the line switch to position 2 provides termination in the repeater'for a second line called hereinafter line 2, which duplicatesmthetermination or line .l including two windings of relay R. These windings are yconnected inline and network branches corresponding to those associated'with line I, butl the wind-` ings are oppositely poled with respect-,to currents .1 from the line. Longitudinal interference curl rents which are equal', in Ythe twolines will, therefore, have practically no magnetic effect on relay R; Theline arrangements are described in detail hereinafter.

9 DIFFERENTIAL DUPLEX OPERATION "Norma and opposite arrangements of differential duplex systems When arranged for diierential duplex operation, the repeater transmits a polar signal from positive and negative 13G-volt battery supply connected to the contacts of relay S. Either polarity may be used for marking. Negative for marking and positive for spacing is called battery normal and the converse is called battery reverse. The windings of relay R must be poled so as to recognize as marking the polarity which is assigned for this purpose at the remote terminal. The winding arrangement which recognzes a remote negative battery as marking is called the line normal condition and the converse arrangement is called the line reverse condition. Three combinations of line and battery conditions are provided in the repeater, permitting two types of differential duplex systems to be set up. One system, Which may be called the normal arrangement, has repeaters at both terminals in the line normal and battery normal condition. The other system, which may be called the upset arrangement has line normal and battery reverse at one terminal and line reverse and battery normal at the other terminal. The rst arrangement results in approximately l total line current when both terminals are transmitting a marking signal. In the upset system maximum total line current occurs when both terminals rtransmit marking signals and approximately 0 total line current occurs when one terminal transmits a space and the other a mark. With this arrangement an intermediate station can be operated on a neutral basis as described hereinafter.

DETAILED DECRIPTION OF TRANSMISSION IN THE NORMAL DIFFERENTIAL DU- PLEX SYSTEM When operated as a terminal of a normal system, the repeater is arranged for the line normalbattery normal condition and must be connected over the line to another suitable differential duplex repeater which is also in the line normalbattery normal condition. Switch OPNI should be in position 0 and switch OPN2 should be also in position 0. This condition is shown on the drawing. Negative 13G-volt battery is connected to the marking contact of lrelay S through contacts ILL-9, section B of switch OPNI and S-I of section A, and positive battery is connected to the spacing contact of relay S through contacts 8 1, section B of switch OPN I. The input signal condition which is to be transmitted over the line is applied to the windings of polar relay S from the local side of the repeater through the balanced loop or the electronic coupling unit. Operation of the armature of relay S in response to the input signal will result in transmitted currents from the local or home battery which will be equal in magnitude but opposite in direction from the marking and spacing signal received from the distant terminal. Component currents furnished by the batteries at the remote terminal of the system will be disregarded temporarily in` order to simply the explanation, The transmitted currents furnished by the home batteries flow through inductance coil LIA and through inductance coil LIB in series with resistance REQI which latter are shunted by resistance R202, through the current regulating resistance, made vtant repeater. .through winding C of relay R, dividing in the 10 up of resistances R3IIA to RIIIA of section A o f the apex switch to the junction of resistances RI3A and RI 3B which is the apex point. Inductances LIA, LIB and resistances RZUI and R202 constitute a noise killer which modies the transmitted signal currents so as to reduce interference in the telephone circuits using the same facilities. Resistance RIBI, which is coninected in series with capacitance CIlI and resistance RIII2 which is connected in series with capacitance CIM, shunt the armature and contacts of relay S and form wave-shaping and contact protective networks for relay S and also protect those contacts of switch OPNI which control the battery supply leads to the relay contacts.

At the apex point, the transmitted signal currents divide, part flowing through winding C of the receiving polar relay R and the remainder through winding D. Winding C is connected through contacts III-,9, section A of switch OPN, contacts I-IZ, section B of switch OPN2, which short-circuit line pad resistors RZA and RZiIB and through resistor R22A and resistor R22B and the top break contact of jack Line, both in parallel with resistor R22A, to conductor Line I, which extends to the remote repeater. Measurement of the total line current can be made in the Line jack. Winding D of relay R is connected through contacts 6 5, section A of switch OPNI through contacts lI--S of section A of the L Pad switch, to the line balancing network of the repeater which is described hereinafter. When the repeater has been adjusted for duplex balance, as described hereinafter, the impedance of the branch circuit from the apex point through winding D and the balancing network to ground closely matches the impedance of the branch through the line winding C, the line, and the remote repeater to ground. The relay windings in the line and network branches are magnetically balanced and oppositely poled with respect to currents to or from the apex point so that the transmitted signal currents, described in the foregoing, will divide equally between the two branches and will have approximately no eiect on relay R, thus the transmitted signals will produce no current iiow vin the ohrn resistor RM bridged between the line and network branches at the end of the llli-ohm resistances RISA and RIBB. A meter connected in jack BalI in series with resistance RI4 can be used to indicate the adjustment of the network to obtain duplex balance since when the network impedance closely matches that of the line, operation of relay S will have no effect on the meter. If the duplex balance is adequately maintained, relay R will be solely responsive to signals received from the distant repeater, so that simultaneous transmission in both directions, or fullduplex service, can be provided it' the local circuits of the repeater are arranged for this type of service.

Signals received by the repeater from the line originate at the distant repeater in a manner similar to that described above. The current components furnished by the distant batteries will now be considered separately, disregarding the components furnished by the home batteries which have no effect on relay R when the repeater is in duplex balance. The signal currents received from the line will be in opposite directions but of equal magnitude for a marking or spacing condition being transmitted by the dis- The received line currents flow 11 "44-.44-.80.ohm delta, formed -by resistances RISA, Ri3B Yand Rlll; adjoining the .apexgpoint .of the repeater, part flowing through the .apex branch includingk the contacts of .relay S -to ground, and part owing through winding D of relay R and the balancing network to ground. .The received current flowing in the network branch is magnetically aiding with respect Ato that in the line branch, and the windings are vso poled that whenrelay S of the distant repeater ison its marking contact,V the armature of .relay R, shown vin Fig. 1,'will be operated to' engage its marking contact. The effect of that partof the recevedcurrent which flows to ground through thel apex branch can be considered in terms of Y -the voltage drop it causes in the apex impedance. This voltage drop applied between the apex point and -ground produces no magneticerfect on relay Rxfor the same reason that the home .battery Vpotential applied to the apex point has no eiect as vdiscussed in the `foregoing. .Adjustment of the apex'resistance can, therefore, vbe 1used -as'a means of controlling the transmitted signal currents without changing the ampere-turn strength of the received signal, which will'remain `constant -at the value it would have if the apex :impedance were zero. Also, the current inresistorRHl, bridged between the line and network branches, will beindependent of the impedance inthe apex branch. This vbridge current, which is dueso'lely to the batteries `at the remote ter minal when the Yrepeater -is in duplex -ba'lanceis 'directionally proportional to the ampere-turn strength of the received signal.

The received signal current is converted 4into sociated local circuits is provided when theconf .ditions of local .terminations are set up. Resistor. RIZI .and condenserCS and resistor R|22 .andcondenser C4 form signal-shaping and conf tact protective .networks for the receiving relay i R. These networks ralso .protect the contacts ofthe. switches in the associatedfbalanced.loop circuit which are in series with the battery'supply loops to the contacts ofzthe R relay.

' .In practice .the total .line current in the normal system may notbe reduced to zero When-batteries of thevsame polarity are connected through the vapex branch Yto the lineat bothterminals. The small line current which may yremain is icallejcl override:current. It may result from unequal I vbattery `voltages or unequal apex .resistancepat the terminals or from line leakage. Over-ride .current has'no eiect on transmission if the repeaters at both terminals-are in duplex balance.

Imperfect duplex balance of the repeater will vresult in unequal division of the transmitted currentsbetween the line and network branches and relay -R will be lsubject to disturbance by the transmitted signal conditions. If transmission takes v-place in only onedirection at a time, which is -thecase in half-'duplex operation, resistance unbalance between the line and network will cause bias. A change which Vresults in lower resistancein .the line, such as would be caused by .-line -leakage to ground, .will tend :.to produce Vsection .A switch OPN! if 'the repeater is in duplex balance.

spacing'bias inthe received signals. .If transmission vtakes place simultaneously with reception, which is the case in full-duplex operation, inequality in the transient line and network currents lwill produce additional distortion `in the vreceived signals.

DETAILED YDESCRIPTION OF HANSA/IIS:-

.SION .IN THE UPSET DIFFERENTIAL .DUPLEX :SYSTEM iAt the line normal.battery lreserved terminal of Yan upse differential duplex system, switch OPN! is operated to position l and switch VOPN2 is operated to the position 0. The switches at the distant repeater are in the line normal battery normal position. Positive 13D-volt battery is supplied to the marking contact of relay S through contacts 8 9, section B of `switch OPNI, and vcontacts .ll-Eil, section A of switch OPN2. 'Negative 13G-volt battery yis suppled'to the spacing contact of relay S through contacts isi-H, section B Aof switch @PNL Otherwise 'the'repeater is identical with the condition v*described for the terminaloffa normalA system. Transmission of signals to the line is accomplished in the same manner as described for a normal differentialduplex system, but the signal currents for transmitted marking and spacing conditions are reversed in direction as compared to those transmitted by the normal terminal.

'Relay P, isnot affected by the transmitted-currents if the repeater is in'duplex Ybalance'as in the case ofthe normal repeater. Receptionof signals bythe lline normal'battery reversed repeater is'accomplished in @the .same manner' as described lunder Athe `normal system, since the received signal currents have the same relative directions for marking'and'-'spacingsignals, and relayR windings are poled in the samev direction as for the normal repeater.

At the line reversedbattery .norma-lterl minal of an upse differential duplex system, switch OPN! is operated to position '2 and switch OPN2 .is operated to .position'O. YThe switches at the distant repeater are inthe line normal bat-- tery Ynormal position. Terminal i or" relayR is' Aconnected to the network branch of the repeater through contacts 5-'3, section A of switch @PNL 'Terminal 6 of relay R is connected to the' line branch of the repeater through'contacts S-2, Windings Vi--Z and 3-5 are thus interchanged between the line and network branches as compared tothe normal repeater described heretofore. Vattery connections and transmission of signals to the yline are the'same as for normal operation. The reversed lline and network windings of the relay R are Amutually -opposing with respect to rthe transmit- `ted Signal currents, and the relay is unaffected Received signal currents iiow in opposite directions for each signal condition as compared to those received from a normal repeater, and therefore are properly recognized by the reversed relay windings, causing relay R to follow the armature of remote relay fS. Y

-An intermediate station in an upset system can be operated lon a neutral providing half-duplex three-way service. The intermediate receiver .follows the line current, which 'is at :maximum value when both terminals are transmitting vmarking' signals, falling to approxi matelygzero when one of the terminals transmits aspacing signal. :A spacing signal is transmitted iat .the intermediate station by .opening :the

line, allowing the network branch current at each terminal to operate relay R to spacing.

A dierential duplex terminal station either line normalbattery reversed or line reversedbattery normal and a neutral terminal station may be operated on an upset duplex basis provided the neutral terminal supplies battery of opposite polarity to the marking battery at the diierential duplex repeater.

DETAILED DESCRIPTION OF THE NET- WORK BRANCH OF TE REPEATER The network branch of the repeater provides for duplex balance of the repeater when used with any of the line facilities. Apparatus units in the line branch of the repeater close to the apex point are duplicated in the network branch. The line facility is simulated by an adjustable line balancing network. In the paragraphs below the line ani network branches are compared in detail.

Beginning at the apex point, one i4-ohm resistor RISA and winding c of relay R are matched by the Lill-ohm resistor RMB and winding d of relay R. Either of these windings may be in the line branch, and the other in the network branch, depending on the position to which switch OPNI is operated. When the line normal connection of relay R windings is required, switch OPN I is operated to position or position 1. Winding c of the R relay is connected through line pad resistor R2 and resistor 52.22A to the line. Winding d of relay R is connected through pad resistors R24A and R24B to the balancing network. When the line reversed connection of relay windings is required, switch O-PNi is operated to position 2, interchanging the connection of relay terminals i and 6 described above. In either case the balanced relay windings are mutually opposing with respect to cur-1 rents to or from the apex branch, and if the remainder of the line branch is closely matched by the remainder of the network branch, relay R. will be substantially unaffected by any voltage in the apex branch. Bridged elements connected and network branches are otherwise in balance,

these bridged elements connect pointsr of equal potential with respect toany voltage in the apex branch, and hence have no current components due to the home batteries and no eiect on the duplex baiance.

Pad resistors R20A and RZUB in the line branch are matched by resistors R24A and R243 rin the network branch. For differential duplex operation both sets of these pads are short-circuited by contacts on section B of switch OPN2 which is in position 0. For pol'arential operation, described hereinafter, the short-circuits are removed by operation of switch OPN2 to either position 1 or 2. The pads can then be adjusted in G-ohm steps from 0 to 450 ohms under control of the L Pad switch. Similar wiring is provided from resistors RZA and RZBB and RNA and R24B to contacts on section A of the switch L Pad so that equal values of resistance are inserted in the line and network branches for each position of the switch. The four positions `of the L Padswitch for 0, 150, 300 and 450 ohms resistance lare duplicated in left-handv and righti4 hand sectors, the alternate sectors -being Iused for switching of the composite set balancing apparatus, as described below.

From the line pad resistors RA `and R20B the line branch of the repeater is extended through resistors R22A and R22B, thence through the line equipment in the oilce to the line facility. If the line is equipped lwith composite sets, these can be simulated in the netconnected work branch lby coil L4 connected in series and. capacitors C5 and C6 connected in shunt to ground. These connections are controlled by section C of switch L Pad. When the switch is set at any of the first four positions for dilerent values of line and network pad resistance, designated CX Bal Out, coil L4 is short-circuited and the lead to capacitors C5 and C6 is opened. When the switch is set rat any of the succeeding four positions, designated CX Bal In, the short circuit is removed from coil L4 and the shunt capacitance is connected. .Iv

The line balancing portion of the network branch includes Ialso a resistance branch and a timing branch which consists of series resistance and capacitance. The adjustable resistance branch provides for balancing the line with respect to direct currents. It consists of resistor R26 in series with the 0-3500-oh1n section of balancing resistance rheostat BRA, connected from coil L4 to ground. The adjustable timing branch provides for approximate balance of the line with respect to transient currents. It consists of the 0-1500-ohrn section of the rheostat TRA in series with a 0-11-microfarad capacitance controlled by the switch C Bal and is connected in parallel with the resistance branch. The variable capacitance is made up of one l-rnicrofarad C8, one 2-microfarad C1, andv two 4-microfarad C9 and C10 condensers, connected in parallel as required. As switch C Bal is rotated, connections are made through sections A to 0, 1, 2, and S-microfarad capacitances in the successive positions, the cycle repeating every four positions. Section B adds O-microfarad in the first 3microfarad cycle, 4-microfarad in ,the second, S-microfarad in the third, thus providing l-microfarad steps from 0 to `11 microfarads in laccordance with the designated positions of the switch. A small resistor is connected in series with Veach of the unit capacitors to protect the switch contacts from excessive initial currents as capacitors are in parallel. These resistors have values inversely proportional to the capacitance in each casefso that smooth adjustment of the timing branch is not impaired. The resistor and capacitance arrangement, and the two functions performed bythe resistor are important features of the invention.

RECEIVING EQUALIZER The present line repeater includes an improved equalizer which is one of the important features of the invention. This equalizer' is in the form of a receiving shunt, including coil L2 and the O-lOGO-ohm section ofv tandem rheostat EQLA which is connected across the line sideof receiving relay windings c and d. This shunt is closed when the rheostat is moved away from the oil position. Small resistors Ri and R16 are for padding out the resistance of the coil when necessary, as described hereinafter. Ifthe repeater is otherwise in duplex balance the `afl-- justment ofthe shunt haswnoeiectvcn trans- :rnttedfcurrentaor on; the..dup1ex balance ofthe homes-epoetin'. .itvshunts fthe: rceci-vingfreies' in a manner which inxampcrefturn:eiectiisrthe A'saine as though' the "apex" :point werefgrounded fandfconnectedzto `the mid-.impedancefpoint of :v5-the shunt aand to `the Vtmid-pointv :of the 8D-.ohm ;;bridge.-;R1I4.

...Because .the :shunt fhas :considerably more :inductance vthan the Arelay windings, it -by-passes .a1relati-vely largerpDr-tion of :the low frequency aand'dinect-.current components ofthe vreceived v,signal:around the relay,.the:degree of discrimiqnation increasing .as the resistance is decreased fbyaclockwise'rotation oitherheostatvThe operating .wave'receivedr byV the relay is vmodiiied in such 2.a 'way `as to. tend :toA`v reduce negative-characteristic distortion. form of distortion .ordinarily .becomes important :in relatively low -gspeedz transmission, isuchA .as vin fio-speed trans- ;missionponly .when the line'. conditions .are especially severe or when theginput .signals are already .disto'rtecl. `"irenefits.to be obtained from rthewequalizer are ytherefore..in the formiof Aincreased zdistortion ',tolera-nce rather vthan improvement iin `norma'loperation with input sig- Analsfof low distortion. At .speeds .of '75 -words r'per min-ute, yimprovement in distortion tolerance *isf greater, `and,therezmay be some reduction in the distortion-:when the .input signals are relaftively undistorted.

YTYPE 'A '-POLAREN'I'IAL "OPERATION Different terminal. conditions required for type A ,polareirtal systems 'In the'type Apolarential system ypolar trans- V'It is -intended-primarily for euse ontcablecircuits. y

TransmissionVVA from. the ltype .A 'polar Sending t0 thetype A .polar receiving repeafe At the'polar sending terminal of.-a typen sys- L'tem switch OPNI .i'sxoperated to-'position and switchOPNafis operatedtothe position l. Trans- 'mission/of input .signals to the .line is .accomplished;by1 the operation of .the `armature .of relayS under A.control of the coupling unit .or balanced loop connected to the local side .of the repeater. Negative 13D-volt battery is connected to the marking contact of the S relay through contacts I .ll-9, section -B of switch OPNl, and contacts |0-9,.section-A of switch OPNZ. Positive 13o-volt battery .is connnected to the spac- ...lng contact of relay S vthrough contacts 8l, .section .B V.of switch OPNI. Sending battery polarities are thus the same as the battery .norma arrangement of a diierential duplex repeater. Polar signal currents, of equal 'value .but inopposite directions for transmitted marking and spacing conditions, ilow through the ,noise killer, the `ap.ex.resistance, and divide between vthe line branch and network branch of the repeater. 'Details of theapex branch are as described for normal differential duplexopersource. When the relay Sis on the spacing conwith no bias resulting from line changes.

iation. The :line branchof thecrcpeaterincludes Winding d of relay R, which is connected through terminals I2-9, section A, switch OPNI, 'through line pad resistors R2 EIA and R2`0B and R2 ZArand R223 to the line. .Series resistance in 15o-ohm steps may be inserted by means of the sections 0f resistor R20 which are controlled by section A of switch L Pad. Measurements of total'` line current can be made in the line jack, .the seetions of resistor R22 being so proportioned that a direct reading in milliamperes is obtained with the meter of a cooperating test set, not shown. If the repeater is in duplex balance, windings of relay R in the line and network branches will have equal currents of opposing magnetic inviluence due to either of the transmitted signal conditions. Relay-R will therefore be substantially unaffected by the transmitted signal condition. Current components in the repeater which `are due to the battery at the 1emotepolar receiving repeater are disregarded at this time in order to simplify the explanation.

vAt the polar receiving repeater of a polarential type A system, switch OPN I is operated 4to position 1 and switch OPNZ is operated to position 2. Winding c of relay R is connected in the line branch through contacts IB-9, section A, switch OPNI which is operated to position 1. Resistance pads RZllA and R20B and meter resistors RZZA and R22B are connected between the line and the relay Winding, .asin the :polar sending repeater. Winding d of relay R is iconvnected between the apex'and the line :balancing network through contacts 6 5, section A, switch OPNI. This arrangement of relay windings is thesame as the line normal condition for differential duplex repeaters. Polar signal currents which originated at the polar sending terminal, as described under the present heading, are received at the polar receiving repeater, flowing through the line winding c of relay R and to ground through parallel paths, consisting of the apex circuit and the network branch. The armature of relay S is connected to ground through its .marking contact. Only a small portion `of the received current flows through the network If the receiving relay R is unaffected by the signalcondition applied to the apex by `its relay S armature. .RelayR windings are so poled with respect to the received signal currents that the Arelay will follow the position of .the relay. S armature `in the remote polar vsending repeater. .If .the polar receiving repeater is not in duplexbalance relay R is not affected While the local relay S Varmature is r on .the marking contact, which is connected to ground, because there are no line and network currentsfcontributed from this tact, which is connected 'to negative battery, .-bias or other distortions in the received signals will result from duplex unbalance, yas in the case of a Adiierential duplex repeater. If half-duplex service, or transmission in only lone direction at a time is provided, relay S will normally be 'in the 'marking position when signals are being received from the remote polar sending repeater so that true polar transmission Will be realized,

Receiving equalizers are provided for useat 'type lA polarential polar receiving terminalswhen -required 'to reduce the characteristic distortion in .the lreceived signals. The ldescriptionand Transmission from the type A polar receiving to the type A polar sending repeater Transmission from the polar receiving terminal to the line is accomplished byioperation oi relay 'S under control 'of the .coupling unit or balanced loop associated with the repeater. Ground is connected to the markingcontact of relay S through 'the RIS resistance, contacts A9-'-I2, section A of switch OPNZ and negative 13G-volt battery is connected to thespacing contact of relay S through contacts [S-H, section B of switch OPN I. In order to facilitate later explanation` of vsignal reception-at the polar Sendving terminal, and the self-compensating 'feature ofthe system, it will bc'considered that `the batn ftery conditions -just mentioned are made up vof a steady component olf .negative v65 volts, which is .the average of the .marking yandV spacing conditions, plus a polar component of positive 65 volts for marking and 'negative 65 volts for spacing. For reasons which are'made evident below,

the steady lnegative 65-'volt component may be called the compensating component of the applied Yvoltage,.and the -volt .polar component maybe called -the "signalingvcomponent `ofthe I.applied voltage. The 'signaling `component agrees in polarity with the batteryreversed diieren- :tia'l v'duplex repeater, but is of only half the-magnitude.

Cornpensating current components `and polar signal current components, correspond*- ing to the respective voltages, are transmitted llthrough the apex circuit, dividing between .the line and network branches without influencing the .receiving relay. because of. duplex. balance. The adjustable resistance in th'e apex circuit, consisting ofv resistors R30A torRlUA controlled by the section A of switch Apex, is .short-circuited through contacts 1i.-5, sectionA, switch OPN2. Otherwise the apex. branch is as describedy for normal differential duplex. hereinbef'ore.

The receiving relay. R` at. the polar sending terminal is connected in. 'the line reversed manner, as stated under the last preceding heading. Current components ld'ue to the positive .and vnegative (i5-volt' polar voltage component applied vby the relay S.y armature in the remote vpolar receiving .repeater will' therefore'infiuence relay R in Aaccordance with the signalconditi'on being transmitted.v rIhe compensating current component. due to. the steady negative .65-volt component of the voltage supplied by the distant repeater` will have a .spacing influence, 'which will be of 'the same rmagnitude asthe 4etlect produced byeither polarity tof the .polarsignal voltage. Biasing current .in a marking direction is supplied to windings .a rand b offrelayl't, .at the polar sending repeater, from positive 13G-'volt battery, through contactsY |'-2., .section A" of switch OPNZ, a nxed voltage-reducing, potentiometer comprising resistorsR, .1R39 and R38 and a variable potentiometer includingresistors R40, RSB, RiiA, R31 'and the Bias rheostat. This cir-cuit has the characteristic of providing approximately constant nneness of control throughout the range of currents required and is another important aspect 'of the invention. The value of the biasing current is adjusted, .utilizing circuit relations described below, so `as to Ajust annui j the spacing finiluence" ofthey com- Pensating current component receivedv from the .remotepolar receiving "repeater: v Current -components from the homebattery willfhavei'no-in- -ue'nce Aon relay VR at the .polar `sendingrepeater so long as the repeater'is in A"duplex ibalance, so that under 'the conditions described 'relay R .fat the polar sendingrepeater will be solely respon'- -sive to the polar signal components "of the :cur-

rents transmitted Yby vthe `'distant repeater.

Adjustment of `relay R biasing icur'rent at "the polar sending terminall is Ibased on :comparative measurements of the `bridge' 'current jack "Bal rvl in series with .resistor .RM connected fbetween the line and'network branches, and tlie current through jack 'Bias inthe bias current circuit. Ifthe yrepeater is in 'duplex'balance ythere :is no current component .in bridge resistor 'RM glue to the home battery, 'regardless of itsvoltageior the impedance in series with the apex. The total current inthe bridge, in the absence' tof :polar receiving line interference, is' furnished byth'eremotieb'atytery and includes a |`snea'dy 'and a polar iconi- 'ponent The `ampere turneffectsfintrelay:due to the steady and' polar components .of theme;- ceived signal currents are 'proportional 'to lthe total bridge current. The total -bridge .current will .ordinarily be zero when the remote relay S is transmitting a mark, sinceY 'the' isteadyA fand polar components are-of equal valueY andrin vop*- posite directions. When .the'fremote `relay .S'i's transmitting a space the sum of thefainpereturn's in relay R, due 'tothe polar and steady :components of the received currents, or vtwicel-th'e ampere turns due to the steady .componentgwill be V.proportional to the bridge lcurrent: rThe biasing ampere turns can vbe made ito'lannul the steady or compensating ampere turnswhenfithe -bias current. is adjustedv to avaluev relative-t0 the bridge current which takes yaccount/:of fthe ratio 'of turns in relay R'line vandv biasing wind'- ings, the current dividing ratio Ybetween'theil'fiohm resistance in series with the .linel winding of relay R and th'ebridge 'circula-'considering the apex point t'o be groundedg and'the 2-to71 ratio. just mentioned between the- Atotaliand the compensating vampere turnsl in .relay' R-"during reception of aV spacingl signal.v Resistors RSSB and R36A' inthe biasing circuit provides :such shunt and series resistance forl a particular test set meter, plugged into jackv Bias; that thebias current will have the-required relationto the bridge current when the biasfpotentiomet'er is adjusted sothat the meter-reading obtained .in jack Bias is the `samefas that-receivedwin jack Bal I during receptionfof aspace. Adjustment .of the biasing. currentvon a vsteady sta-te 'basis as` described kabove is only an approximation. Final adjustment` should `be .made on the basis of. minimum received .distortion as indicated' by transmission measuren'ients, thus providing .for `a mop-up of 4distortion due to apparatus variations,lcircuit characteristics and inac'curacies'in meter readings or control settings.

The self-compensating' action whichv is .Drovided "to minimize the effector changes inline resistance on transmission in the differential direction, that is, vfrom the polar receiving 'to -the polar sending terminals, can be explained' lby considering separately the various voltages which are operating in the system. The' polar current components due to thesignalin'g voltage' transmittedfrom the polar receiving vterminalA 'will :be modied by any change in the linie, n'-but' since marking and spacing signal currentsare iaiected in the Asame rdegreefno lbias will be caused Virrt-he signalsk received .at thek polarsendir'ig repeater. Assuming that halflduplex transmission `is bein-g provided, relaylSiofthe polar sending repeater terminal.

will rest on the marking contact'while signals are'being received by relay R of Vthat repeater. Negative battery potential from the home .battery of the polar sending repeater will be applied through the apex circuit, producing current components in the line winding d of relay R which have a marking influence and current components in the network winding c which have a spacing inuence. Under the conditions of duplex balance these currents are equal and the resultant magnetic effect is zero, as discussed previously, but if the line resistance increases from the condition of initial adjustment the network current will over-balance the line current and a spacing influence will be developed, tending to produce bias in the received signals. The increase in line resistance will also cause a -decrease in the compensating component of the current transmitted by the distant repeater. Since this component has a spacing influence, a decrease in value will be in the marking direction to compensate for the spacing effect in relay R due to duplex unbalance. Conversely, if the line resistance had decreased, the duplex unbalance would produce a marking influence in relay R and the increase in the spacing compensating current would tend to counterbalance this effect, limiting the bias which may develop to a small amount.

Exact compensation for changes in line resistance will be obtained only when the steady negative marking voltage at the polar sending terminal is reduced in the potentiometer formed by the apex and network branches, to such a value that the sum of the line current components in the marking condition due to this voltage and the compensating voltage at the polar receiving terminal is zero. Practical adjustment of the type A polarential system is based on uniform total line current, in the interests of simplicity. Line pads are inserted in the shorter lines in order to obtain qualitative agreement with the conditions for best self-compensation, as mentioned above. On the longer lines the polar sending apex resistance is less than the value required for best self-compensation, and it may be desirable in some cases to increase this resistance at the expense of line current. If the apex of the polar sending repeater is adjusted to the exact value for perfect compensation, as described above, and there is no line leakage, transmission of signals from either terminal will result in equal values and opposite directions of total line current when marking and spacing signals are transmitted, This does not correspond to equal ampere turn strengths of the signals in the two directions, because of the characteristics of the receiving circuit heretofore described, but it is a desirable condition since equal ux densities are produced in line apparatus for the two signal conditions.

The self-compensating action described above is dependent on relay S at the polar sending terminal remaining operated to its marking contact, and is therefore of benefit only when half-duplex service is being furnished. If relay S of the polar sending repeater is operated to its spacing contact while signals are being received from the line the eiects of any duplex unbalance caused by a change in line resistance will be exaggerated instead of compensated for by the steady compensating voltage component at the polar receiving Distortion in the signals received by relay R will therefore be greater than in the case of a differential duplex repeater with the same degree of unbalance.

This will be of no practical consequence in half-duplex'service unless the effect on relay R is large enough to impair the operation of the break or double-space bypass features provided by the associated balanced loop or coupling unit, or unless false operation of relay R, or kick-off occurs during normal transmission of outgoing signals. 1f the duplex unbalance is due to line leakage, its eiiect will be exaggerated by the compensating voltage component when relay S at the polar sending terminal is on its marking contact, which is the normal condition during transmission in the diferential direction. For this reason, type A polarential operation is unsuited to lines subject to appreciable leakage.

Receiving equalizers, heretofore described, are provided for use at type A polarential polar sending terminals when required to reduce the characteristic distortion in the received signals. The functioning of the equalizer as regards modification of the received signal is the same as in the diierential duplex case. There is an additional effect at the polar sending terminal of the polarential system, because of the reduction in the steady or compensating ampere turns in relay R. This reduction is determined by the relative resistance of the parallel direct-current paths through the equalizer and the relay windings. Each value of the equalizer resistance therefore results in a different value of the bias current required to annui the steady or compensating ampere turns. However, the relation between the bias current and the current through jack Bal l remains the same as described above, regardless of the equalizer adjustment, provided that the repeater is in duplex balance.

TYPE B POLARENTIAL OPERATION Dierent terminal conditions required for type B polarential systems Type B polarential systems, like the type A systems, employ polar transmission in one direction and differential transmission in the other. Complementary arrangements of the repeaters at the two terminals are required, one being designated as the polar sending repeater and the other as the polar receiving repeater. Battery and line connections in each repeater are set up under control of switches OPNI and OPN2. The system provides half-duplex service and has the characteristic of being aiected to a relatively small degree by line leakage. It is intended primarily for use on open-wire lines.

Transmission from the type B polar sending to the type B polar receiving repeater At the polar sending terminal of a type B system, switch OPNI is operated to position 0 and switch OPN2 is operated to position l. Battery connections to the relay S contacts and the connection of relay R windings in the line and network branches are completed in the normal manner as described in detail for the terminal of a normal differential duplex system. Biasing current in a marking direction is supplied to windings a and b, of relay R from positive volt battery, through contacts I-2, section A of switch OPN2 to the voltage-reducing potentiometer circuit previously described. Operation oi relay S armature in response to the input signals will result in the transmission of polar signal currents through the apex and the line and net-.- work branches in the same manner as in the differential duplex system. Likewise, relay R will for leakage betweenthe compensation point and the polar sending or polar receiving terminals, respectively. Adjustment of the polar sending apex resistance is designed to x the compensation point approximately in accordance with the requirements mentioned above. In order to obtain this condition on the shorter lines it is necessary that the line current be limited by the line pads, instead of by the apex resistance. Longer lines require higher values of apex resistance in order to properly rlx the compensation point, but this reacts unfavorably on the line current, thus limiting the resistance of the line over which type B operation can be used.

Best performance can be obtained from type B polarential systems by modifying the initial adjustment of self-compensation as experience indicates the need. This is particularly true if initial line-up was made during the presence of line leakage. As mentioned, leakage near the polar sending terminal tends to cause spacing bias in the signals transmitted in the differential direction, while leakage near the other end of the line causes marking bias. A system lined up during either of these conditions is subject to the widest possible swing in line conditions. The self-compensating action will tend to reduce the bias which may occur, but this will be greater than if the system had been lined up during a more nearly average condition of the line. Either a line with no leakage, or a line with uniformly distributed leakage over its entire length can be taken as the median condition. The system can be adjusted under either line condition with nearly identical results as regards the maximum amount of bias which may develop because of non-uniform leakage on the line. It is more practical, however, to determine when the line is free from leakage than to determine when the leakage is uniform or normal. It is preferable therefore to make the iinal adjustments of the system when the line is dry or free from leakage, rihe distribution of leakage which occurs during wet weather conditions on a particular system may be such that a predominance of bias of one sign will be experienced. If there is a predominance of marking bias, it indicates that the effect of duplex unbalance is smaller than the change in the compensating component of the received signal currents. Improvement can be made by decreasing the apex resistance, thus increasing the eiect of the existing unbalance without changing the effective strength of the steady and polar signal components received from the polar receiving terminal. Conversely, a tendency toward spacing bias may be corrected by an increase in the apex resistance.

When transmission is in the differential direction, the total line current is in the same direction for both signal conditions, but roughly 21/2 times greater during the transmission of a spacing signal. The relatively large and unequal ux densities which may result in magnetic apparatus may contribute to distortion in the system, but these effects are more or less mopped up by the adjustments which are provided for obtaining minimum total distortion.

As in the case of the type A polarential system, the self-compensating feature is dependent on relay S at the polar sending terminal remaining on its marking contact, and is therefore eiective only when half-duplex service is being furnished. If relay S is operated to its spacing contact while signals are being received, the effects of duplex `unbalance due to line leakage will be exaggerated instead of'compensated for by the change in the steady component of the received current. Distortion in the signals received by relayR will therefore be greater than in a differential duplex repeater with the same degree of unbalance. This will be of no practical consequence in halfduplex service unless the eifect on relay R is large enough to impair the operation of the oreak or double-space bypass features provided by the associated balanced loop applique or coupling unit, or unless false operation of relay R, or kicko1'occurs during normal transmission of `outgoing signals. If the duplex unbalance is due to a change in series resistance of the line, its effect will be exaggerated when relay S armature is on its marking contact, that is, in the normal condition during transmission in the differential direction. Because of this characteristic of the type B system, it is best suited for use `on lines on which the greatest changes are those due to line leakage.

Receiving equalizers are provided for use at type B polarential polar sending terminals when required to reduce the characteristic distortion in the received signals. The description and functioning of the equalizer as regards modiiication of the received signal is the same as covered for differential duplex operation. An additional eiect at the polar sending terminal makes the required value of bias current dependent upon the equalizer setting, as described for type A polarential operation. Y

DESCRIPTION OF PROVISIONS FOR DIF- FERENT LINE ARRANGEMENTS Adaptation of the repeater to various line arrangements is controlled by the Line switch. The line arrangements are related to the amount and kind of line interference, and are largely independent of the type of transmission employed.

One-wire operation For operation over a single telegraph line circuit with a low level of interference, the Line switch should be set in position 0. Relay windings e and f employed in the termination of the No. 2 or neutralizing line .as described below, are opened at contacts l and It, section A of switch Line, so that even if a second line were connected to Line 2 of the repeater, no complete circuit through the relay windings would be provided. Miscellaneous .interconnections occur among the apparatus units employed in the No. 2 apex circuit or the SO-cycle shunt, but there is no complete circuit which affects the operation of the repeater.

One-wire operation with 60-cycle receiving shunt nation of the No. 2 line circuit as described below. l

When the switch Line is in position l, one terminal of each of the windings of coils LSA, LGB L1A and L1B is connected in parallel through terminals 2, 3 and Ii of section A of switch Line and-l the' opposite terminals iof" each coil are; connected in a'manneiuto formV a'common-,point through contacts ily and1l24 of section A and'contacts I and H of section B of switch Line. The four'coils thus joined in parallelare connected in series with capacitances CI'4', C15 and `CIB which aggregate about 7y microfarads capacitance, one terminal of each of which is connectedto the parallel coils through contacts l, 2 and iQ of section B of switch Line and the opposite terminal of each of which condensers is'connected through terminals 4 and 5 of section Bof switch Line. This combinationis bridged across windings c and d of relay- R, Ywhich bridge is closed by connecting winding cto terminal 5 of section B of' switch Line and. winding d to terminal s of section A of switch Line.`

When connected as described, and with the repeater in duplex balance, the shunt current' will be independent ofthe voltagea'nd impedance in series with the home apex'. The ampere turn eiiects in relay R due to the voltages in series with the line will be the same as would result if the mid-impedance point' of the shunt were connected directly to the mid-pointsl of other bridged'elements, the'ape'xl point, and to ground. This relation follows from the characteristics of the balanced line and network circuits, as described for differential duplex operation. The direct-current component ofthe received signal is not affected by thefGO-cyole shunt, and other frequency components required for telegraph transmission are only slightly aiected.

Twofwir'e operation Maximumreduction in thefelects of all kinds of` line interference, including that dueto power induction, earth'pote'ntialsfand telegraph crossre is obtained byft'wo-wire operation of the repeater. This requires'the provision'of a second or neutralizing--telegraph line circuit, as nearly as possible identical with thefNo. l or transmission'line circuit. When switch Line in therepeater is operated to position 2, the-'No.- v2-line'is terminated in the rrepeaterlin anfimpedance similar to that provided inthe`VV termination ofthe No. 1- line. ThefN'o.' y2line tern'iir'iation includes line, apex, and network branches, and also bridged velements between the line' and network branches, which duplicate the corresponding parts associated with? the No; l linecircuit; Hereinafter the designations No. l and No. 2j are used for convenience in referring to correspond ingelements in theterminationof the No.1 or transmission line and the No. '2for neutralizing line. All variable elements in therepeater'which are duplicated in the terrninations'Yof the' two lines, are tandem l`a` ssembled as single controls in orderto'maintain the equivalence of the two line terminations under' all conditions` of;repeater ad;V justmenft. Line andvnetworklwindings of relay R in the No. 2 line termination are mutually opposingwith respect to'currentsto or from the No. 2 apex branch, mutually-aiding with` respect to currentsrto orvfromthe l \loj2 linie, and o ppositely poled with respect the irelay windings `associated with the No. 1 line as regards currents in the same direction in both lines. It isV evident that if the two lineshave substantiallytl'iesame` coupling to the source of theinterference, equal 'current components will 'be produced in the two lines and their respective terminations, and the resultant effect'on relays R at 'bothterminals will benil. Thiszcon'dition of the systemfniay be called longitudinal balance. If'the'No;v llin'e 26;'. isin duplexbalance` with its network,as fdef scribedV underf the description- Vofthe network branch; the No. 2 line and networklcaranclfies;will

also be; in duplex balance. ,Il-Iowever,4 longitudinal :balance vcan be providedgwithout duplex balance so long'r asthe two linesand theirtermina-A tions are identical. In the following'rparagraphs thextermination of" the-No.2 line4 circuit is com]- pared inidetail with the termination of the No. lline circuit. 1 n :L

Operation of switch Line; toA` position 2 transfers capacitorsv Citi, Cl and Clifrom their a1- ternate connectionin :the C30-cycle` shunt -to the No.y 2 line.' balancing network; as describedchereinafter andv transfers-coils Linerv and LSB" from the llecycleishunt to the` No.` 2 apex branch. Ground .is suppliedY through contacts 3-'-I 2, rsec? tion -B-,of switch ,Line to resistor y.lt4l.which matches thebattery tap in thefNo. 1 apex;` AResister Rel is `connected throughcoilfLtB, contacts' 2'E i, section Aoithe switch,Line,.,through-coil LiiA, the resistor RMA, .resistor 42B f and terminal l or section A'of switch Line. ,Terminals l, 2 and il of section'A ofl switchiLineiconstitute a common electrical point for this condition. Coils LA and LtB and resistors RAZA and R42B'thus iorin a duplicate lof thenoise killer in theNorl apex circuit, which is. madeup ofgcoilsfLlAand LIB and resistors R20I and1R22. The"'J'un c tion of resistors R42A and vrRMB '.lisy'connectedthrough resistors RI IlAito RBGA.: thence to` the' mid-point of resistors'RMA and R433, whichgs the No. 2 apexpoint. Resistors Rli'A'to RI'IOAv are wired to contacts on section' Bofthe Apex switch,V duplicating the' relationsy between` resis-` tors RSeA to RlEA. and sectionA of the Apex switch, thus providingequal values'of seriesre-eA sistance in the No. 1 and No. 2'apex kbranches for all settings of the switch. Resistors RDA to Riten are short-circuited through contacts 1 -8, section A of switch OPN2 when this switch is inthe positionZymatching the short circuit which is placed ,across resistors R'llA to R'lA by contacts -5 on the same section when switch OPNE is operated to position 2, as required-at the polar receiving terminal of;a type A or B polarential system. The resistorv deltal consisting: of the RSA, Rl-iB and'RM adjacent to the No. 2 apex is completed by resistor R44 in series--with jack Bal? connected across the Vends'of theresistors RMA and s RMB. This arrangement matches the resistors RISAgRBiand Rllland jack Bali in theNo. 1 apex circuit.I An equalv icing shunt, including coil'LB vand* thev 0+1000- ohm B section of tandem rheostat -EQLB is connected between the lower: or line terminals of winding c and f of relay R, matching a; similar shunt made up of coil`L2-and section A of the EQLA rheostat, whichis ccnnected'between'the line terminals of windingsc and d.- The small resistors in series with coils L2A and L3 canbe strapped so as'to padout .the two-coilsj towithfin 5 ohms of the samevalue, thus maintaining longitudinal balance. y 1 f,

Current in the .bridgedjresistor Rll'll inthe No. 2 line terminationzbears the sarne- 3ampere turn relation in windings -e and fof'relay R1 thatcurr rent in resistor Rlfi'rloes'to the ,ampere turns due to current in windings cand d, as described for the normal differentialduplex system. When the repeater is in duplex-and longitudinal balance, any voltage'in series-with the No. 2 apex branch will produce no'currentgin resistor R@ and no magnetic4 effectjngthe'relayR. l ,'Ifhe total currentv in resistor R44,I un'derthe` conditions mentioned, is due to the voltage in series with the No. 2 line, and the ampere turns in relay Rare directly proportional to the current produced by this voltage. However, a, given current direction in jack Bal2 indicates relay ampere turns which are opposite in sign to those indicated by current in` a similar direction through jack BalI. It is evident, therefore, that a dierential measurement of the current in jacks Ball and Bal2 gives an indication of the total or resultant ampere turns in relay R due to currents received from lines 1 and 2. Such a method of measurement can beY used in two-wire systems to cancel the effects of line interference when the duplex balance is adjusted to give minimum bridge current. A differential measurement of the Ball and Bal2 currents can also be used in a preliminary setting of bias current.

'Relay R winding f is connected in the No. 2 line branch and winding e in the No. 2 network branch of the repeater, or vice versa, under the control of switch OPNI. The various positions of this switch also provide line normal or line reversed connection of windings c and d to the No. 1 line and network branches as described for the various transmission systems. In each position of the switch, windings f and e are connected in such a manner as to be magnetically opposing to windings c and d with respect to currents in the same direction in the No. 1 and No. 2 lines. In positions 0 and 1 of switch OPNI, terminal 8 of winding f of the relay is connected through contacts 2-I of section A to the No. 2 line circuit and terminal I of the relay is connected through contacts 2-I or section B to the No. 2 network circuit, matching the line normal arrangement of the No. 1 line and network windings. When switch OPNI is in position 2, the connections to these relay winding terminals are interchanged, thus matching the line reversed connection of the No. l line and network windings. Although relay windings e and f are thus properly poled for longitudinal balance with respect to windings d and c for any position of switch OPN, the No. 2 circuit is completed only when switch Line is in position 2. The No. 2 line branch is extended from contact I, section A of switch OPNI, through pad resistors R2 IA and R2 IB and resistor R23 to the No. 2 line. The No. 2 network branch is extended from contact I, section B of the OPN! switch, through pad resistor R25A and R25B to the No. 2 line balancing network. Section B of switch L Pad is wired to No. 2 line and network pad resistors RZIA and R2IB and R25A and RZEB in the same Way that section A of the switch L Pad is wired to the No. 1 line and network pad resistors RZGA and RNB, RMA and R245. Thus, when the switch L Pad is operated to any position in order to provide series resistance in the No. 1 line and network branches, identical values of resistance are inserted in the No. 2 line and network branches. When differential duplex operation is being used, pad resistors R20A and RZIBB, RMA and R243 in the No. 1 line and network branches are short-circuited through contacts I2-I and 9-Ii, respectively of section B, switch OPN2, which is in position 0. At the same time contacts 3-41 and 6-1 of the same section short-circuit resistors RZIA and RZlB, R25A and R25B in the No. 2 line and network branches. From the line pads, the No. 1 and No. 2 line branches of the repeater are extended through resistors R22A and R23 respectively, to the No. 1 and No. 2 lines. The No. 1 and No. 2

network branches are extended to identical line balancing networks, as described below.

The No. 2 line balancing network includes composite set balancing apparatus, a resistance balancing branch and a timing branch which duplicate corresponding elements in the No. 1v network. Coil L5 and capacitors CI5 and CIB, controlled through section B of switch Line and section D of switch L Pad constitute the com posite set balancing apparatus-of the No. 2 network, corresponding to coil L4 and capacitors C5 and C6 in the No. l network, controlled through section C of switch L Pad as described. Capacitors C15 and Clt` which are used alternately in the 60-cycle shunt as described in the foregoing, are transferred to the control of switch L Pad, through contacts IZ-I and 5'-'I, section B of switch Line. Resistor R27 and section B of tandem rheostat BRB form the resistance balancing branch of the No. 2 network, matching R26 and section A of the rheostat BRA which form the corresponding branch of the No. 1 network. Sections A and B of the tandem rheostat TR are the resistance elements of the timing branches of the No. 1 and No. 2 networks, respectively. 'I'he 0-11 microfarad capacitance in the No. 1 timing branch made up of capacitors Cl, C8, C2 and CIO is duplicated by a similar capacitance made up of capacitors CM, CI I, CI2 and CIS in the timing branch of the No. 2 network. The latter group of capacitors is controlled through sections C and D of switch C Bal in a manner identical with the control of the former group through sections A and B of the same switch, as described heretofore. Capacitor CIA, which is used alternately in the (iO-cycle shunt as described is transferred to the control of switch C Bal through contacts 2--I2 and 8 9, section B of switch Line. The small protective resistors in series with each of the capacitors in the No. 1 timing branch are also duplicated in the No. 2 circuit.

The No. 1 and No. 2 line terminations, as described above, are completely independent of each other except for the mutual inductances among the relay windings. These mutuals and possible mutuals between the No. 1 and No. 2 lines may result in slightly different adjustments of the balancing network for optimum duplex balance for one and two-wire operation. However, the diierence is likely to be so small as to be of no consequence except in full-duplex differential duplex operation. There may also be a slight diierence in the optimum equalizer adjustments for one and two-wire systems. This follows from the effect of the relay mutuals, coupling the No. 2 circuit to the No. l or transmission circuit. The effect on the received signalis the same as though the inductances of the No. 2 line and network windings of the relay were added to the inductances of the No. 1 line and network windings, changing the shunting ratio between equalizer and relay windings. However, the diierences in transmission between one-wire and two-wire systems, aside from the eiects of interference, will generally be negligible.

BALANCED LOOP CIRCUIT The left-hand portion of Fig. 1 which comprises essentially the balanced loop switch Loop and the break relay BK together with the associated resistors, jacks and battery connections, is employed when the line repeater is to be connected on its local side through a balanced loop. Such a connection is usually terminated on its 

